It is well known that stimulated Raman scattering (SRS) in optical fiber can, in principle, be utilized for amplification of signal radiation as well as for a laser. Stimulated Brillouin Scattering (SBS) and four-photon mixing in optical fibers are also known. See, for instance, "Optical Fiber Telecommunications", S. E. Miller et al., editors, Academic Press 1979; pages 127-133, 133-135, and 140-144, respectively, all incorporated herein by reference. See also "Tunable Lasers", L. F. Mollenauer et al., editors, Springer Verlag, pp. 279-301, also incorporated herein by reference. Optical fiber lasers and amplifiers based on SRS, SBS, or four-photon mixing will herein collectively be referred to as "non-linear interaction" (NLI) lasers or amplifiers, as the case may be.
To the best of my knowledge, NLI lasers or amplifiers have so far not been used in optical communication systems.
One of the reasons for the neglect of NLI lasers and amplifiers is the difficulty of achieving in the optical fiber the required high CW pump radiation intensities. For instance, with regard to SRS, much of the relevant prior art involves the use of pulsed pump radiation. The obtainable high peak power of the pump pulses results in substantial SRS despite the relatively low efficiency of SRS. Pulsed pump power is, of course, not useful for, e.g., signal amplification in an optical fiber communication system, or for a continuous wave (CW) laser. For example, use of a pulsed amplifier would require synchronization of the pump laser to the signal. This is not only impractical but also would negate one of the main advantages of optical amplification, namely, the possibility of changing the signal transmission rate without making any changes in the amplifiers.
In view of the many desirable characteristics potentially possessed by NLI amplifiers and lasers, it would be desirable to find a way to, inter alia, increase efficiency such that CW pumping becomes practical. For instance, it would be desirable to have available NLI devices that can efficiently utilize CW pump radiation from commercially available laser diodes or laser pumped solid state diodes to produce high intensity CW radiation of a wavelength that is of interest for optical communications (e.g., 1.55 .mu.m), or to amplify radiation of a wavelength (e.g., 1.3 .mu.m) for which other convenient optical amplifiers do not exist. This application discloses such devices.
High power laser sources of CW radiation of appropriate wavelength (e.g., 1.06 .mu.m) are known. Exemplarily, a Nd:YAG laser is pumped with the high power 0.8 .mu.m output of an array of AlGaAs laser diodes. Whereas the output of such a diode laser array cannot be efficiently coupled into a single mode optical fiber, the output of the Nd:YAG laser can readily be coupled into such fiber. See, for instance, S. Grubb et al., Electronics Letters, Vol. 28(13), p. 1275.
M. Nakazawa et al., Journal of the Optical Society of America, Vol. 1(1), p. 80 discloses a Raman amplifier that uses a pulsed (1.06 .mu.m) YAG pump laser to amplify 1.3 .mu.m signal pulses and reports a gain coefficient of 2.0.times.10.sup.-12 cm/W. V. I. Belotitskii et al., Soviet Journal of Quantum Electronics, Vol. 20(7), p. 753 report Raman amplification of 1.24-1.3 .mu.m radiation using a Q-switched YAG:Nd.sup.3+ laser (1.06 .mu.m). P. N. Kean et al., Journal of Modern Optics, Vol. 35(3), p. 397, disclose an optical fiber Raman oscillator that uses a pulsed (mode-locked) Nd:YAG laser (1.06 .mu.m). In one embodiment, the laser cavity is defined by two etched fiber gratings. The output radiation had a wavelength of about 1.09 .mu.m. C. Lin et al., Optics Letters, Vol. 1(3). p. 96 disclose a Raman oscillator that uses a CW (Nd:YAG; 1.06 .mu.m) pump laser. The Raman laser cavity is defined by means of mirrors. The output had wavelength in the range 1.08-1.17 .mu.m. F. Irrera et al., Journal of Applied Physics, Vol. 63(8), p. 2882 observed SRS in silica-based optical fiber, using a CW Nd:YAG (1.06 .mu.m) pump laser. Only the first two Stokes lines (1.12 and 1.18 .mu.m) were observed, with the power in the second being only about 4% of the power in the first (for 5.7 W pump power). FIG. 7.8 of "Tunable Lasers" (op. cit.) shows the wideband continuum radiation generated in silica fiber pumped with a Q-switched Nd-YAG laser at 1.06 .mu.m. And C. Lin et al., Optics Letters, Vol. 6(10), p. 493 (incorporated herein by reference) report four-photon mixing in single mode optical fiber using a Q-switched Nd:YAG laser at 1.319 .mu.m.